The benefits of using elastomeric hydrophylic open-celled stabilized soil modules or plugs for a rooting medium are well-documented by commercial application in the production of transplants, rooting of cuttings, and as an adjunct step in transplanting of field-grown seedlings. Approaches to these applications are many and varied. For example, lettuce transplants which are greenhouse grown for 12 to 15 days after seeding in plugs (1.25 cm in diameter and 4.5 cm in length) will, upon field transplanting, usually greatly outyield either direct seeded lettuce or lettuce grown from transplants which are greenhouse grown for 25 to 35 days in a nonstabilized medium. Further, plug transplanted lettuce fields are more uniform in maturity, requiring a fewer number of successive harvests to obtain the maximum yield.
Celery transplants which are initially greenhouse grown in stabilized soil plugs for 25 to 40 days will at harvest often yield more than corresponding celery transplants which are grown for 50 to 75 days in nonstabilized medium. Like lettuce, these higher yields are attributable to not only the stabilized medium per se but also to the fact that it permits the use of younger transplants. Fields grown from these younger plug-grown celery transplants tend to form plates at a more uniform depth than other transplants. With use of automatic cutting and harvesting of the crop, both undue retrimming before shipping due to too deep cutting or undue loss from shattering due to too shallow cutting are minimized.
The transplanting of physiologically younger and morphologically less developed plants does not result in a correspondingly longer time in the field between transplanting and harvesting since transplanting shock is virtually absent in properly handled plug-grown transplants. Generally, plug-grown lettuce transplants will be ready for first harvest zero to three days later than corresponding lettuce transplants grown in nonstabilized medium which are 10 to 20 days older at the time of transplanting. Plug-grown celery transplants, usually about a month younger at the time of transplanting, will be ready for harvest zero to 10 days later than corresponding older transplants grown in nonstabilized medium.
The absence of transplanting shock with young seedlings grown in stabilized soil plus is the consequence of a number of factors. The root system is less fully differentiated so that more roots are hair roots and water absorbing. The roots are substantially preserved and protected by the medium during transplanting. Aerial portions of the plant are not as extensively developed and thus do not put a great water demand upon the root and stabilized soil system.
The provision of a soil plug enabling transplanting of younger plants has yet another advantage. This advantage derives from the fact that during the short growth period under controlled environment, these juvenile plants are not as greatly morphologically modified by sophisticated greenhouse environments, are physiologically "harder" plants and are thus more easily transplanted. This advantageous difference is evinced in one example by the fact that these juvenile plug-grown transplants set in the field at a 45.degree. angle will straighten and grow upright without either deleterious or noticeable deformation of the mature plant.
A desirable medium which has been used for stabilized soil plugs is formed by mixing a urethane prepolymer with a slurry composed of lime amended peat and water and permitting it to foam. However, forming the soil plugs of this material has presented an ongoing problem.
Large soil masses may be formed by the urethane foaming reaction and the soil modules desired may be cut from the soil mass. However, despite the relatively short pot life of the prepolymers employed (30 to 300 seconds as used), large sized soil masses will have visibly differing densities and structure from the top to the bottom of the soil mass. This condition results in demonstrably different physical characteristics of plugs or modules, depending upon where they are cut from the soil mass. Also, some waste from trimming occurs with such a molding system. Another alternative for forming the soil plugs is to place the freshly mixed slurry and prepolymer directly into a mold of the desired size and shape and permit it to foam in place. However, the material as mixed is quite viscous and does not flow well. Thus the filling of small bore long cavities is very difficult both due to problems of flowing and air entrapment. Further, if such cavities are filled in a vertical position, a density gradient is formed along the cavity yielding a plug of different physical structure and density from top to bottom.
Another molding method is "pan forming." This method consists of filling a pan with prepolymer slurry mix and then placing a multiple cavity mold into the pan, forcing the prepolymer slurry mix to flow upward into the mold cavities while foaming. Though this method may effectively solve the air entrapment problem during mold filling, it is wasteful. Overfilling of the pan is necessary to ensure that all cavities of the mold are completely filled because of both uneven distribution in the pan and lack of precise control of the foaming reaction. Thus though waste in a pan-molding system can be minimized, it cannot be eliminated.
Specifically the soil plugs obtained from the foaming reaction when properly manipulated are a body of elastomeric open-celled hydrophylic rooting medium with a quantity of soil particles being held as an integral part of the foam matrix. The plugs are used both to germinate the seed and also to transplant the resulting seedling while it is still quite small. Due to the special physical characteristic of the soil plug, there occurs an integral penetration of the roots within the medium. The seedling must be transplanted without removing it from the soil plug because roots and medium behave as a single system. The encasement of these tender roots within the elastomeric medium eliminates damage to the roots which would occur if the seedling were either removed from the soil plug or transplanted by other means. Damage to the aerial portions of the seedling during transplanting is similarly avoided inasmuch as the soil plug itself is handled rather than the seedling top.
In order to efficiently handle soil plugs, it has been desirable to form such seed plugs in an interconnected array. Heretofore, the soil plugs in chains or belts have been formed by extending a continuous strip of porous material through the plugs, the plugs being molded and cured around that material. The extension of that material through the soil plugs has however tended to reduce the strength of the soil plug since the plug is in effect longitudinally bisected by the connecting material. As a result, soil plugs so made have been more susceptible to breaking apart. These plugs have also had the disadvantage of not being interconnected over their full length since the end of the plug in which a seed cavity was formed could not accommodate the internally disbursed attachment member. Inadvertent twisting of the chain of plugs has thus occurred. Further, the centrally deployed interconnecting member has rendered the plugs somewhat inapplicable to some root crop production by, in some cases, restricting extensive root growth to but one side or the other of the interconnecting member. A long central root is desirable in plants, such as sugar beets, in which the root constitutes a storage organ and the desired plant part. Branching of the central root ("sprangling") results in a yield loss. Further, roots in long thin plugs may be transplanted more deeply in the field, thereby aiding in survival of the plant since roots emerging from the bottom of long plugs are in contact with soil moisture not readily lost to surface drying.
Some cultural practice constraints have arisen due to the use of prior art soil plugs. When prior art plugs were transplanted into dry surface soils, they have had water withdrawn from them. This condition has imposed the requirement of irrigation immediately upon transplanting and subsequent maintenance of adequate surface soil moisture to prevent drought damage until the transplants are deeply enough rooted so that surface drying will not desiccate plug and plant alike. Further, if the plugs were not planted deeply enough, the top of the soil plug sometimes acted as a wick, removing water from around plant roots and from the surrounding soil by evaporation from the upper portion of the exposed plug.
The present invention is directed toward overcoming the cultural and mechanical problems set forth above.